Why does Serenity have a cannon in “Serenity”?

Question

In Firefly (episode: train job), Mal gets in a bar fight because he was a Browncoat. He gets thrown out of the bar, and the gang he was fighting follows him. They were about to kill him when Wash brings serenity up and tells the gang to leave, unless they want to get blown up.

"Every man there go back inside, or we will blow a new crater in this little moon."

The gang leaves, and Jayne laughs and says

"Damn yokels can't even tell a transport ship ain't got no guns on it. (laughs)"

but then, in Serenity, mal says

" Jayne, you and Wash hoist up that cannon mount. Goes right on top."

I know that in Serenity, some time has passed since the last episode of Firefly. Is there any canon evidence (like from a comic book, or something Joss said) that explains why they suddenly have a cannon?

Answer 1

They attach it during Serenity, when they go into Reaver space.

During the final act of the Serenity film, the crew disguises Serenity as a Reaver ship, including attaching corpses to the hull and painting it in fake blood.

As part of this effort, they take a cannon from the colony Shepherd Book was staying. Presumably it's for defense in Reaver space, though they also use it to "get their attention" later and inspire the Reavers to chase them.

Here's the sequence showing exactly where the flack cannon came from:

Mal running toward cannon and Shepherd Book

Shepherd Book dying on the sandbags of the cannon

Serenity crew preparing to attach the cannon to the top of Serenity

Here's a a high-quality image that makes it clear that the cannon on Serenity in Reaver space is the same one from the planet. Also note the tie-lines and braces, from the quick-and-dirty job of attaching it to the hull.

Fun Fact: The prop used in the film is actually a genuine World War II-era German Flak 30/38 20mm anti-aircraft gun.

Answer 2

According the the film's official novelisation, the cannon belonged to the community of Haven where it was used as a civilian defence tool;

Sadly, the town proved a powerful reminder to Mal of Lilac—several shacks, not as ramshackle as on the other moon, but indicative of the same hard living. They also had a church, which got more use nowadays, and a smal vegetable patch, which was another of Book's contributions to the community.

One thing they had that Lilac didn't—and could rightly have used—was a large cannon, salvaged off a warship some years back. It was the main thing that kept Haven living up to its name.

Dozens of townsfolk came out to greet them, including Doane, who had cannon duty today. He jumped down from the chair that had been attached to the cannon and gave Mal a hearty handshake. Little Hiroko ran toward them at top speed, yelling.

It's also explained that this is the cannon that Book used to shoot down the raider.

Without another word, Book ran over to the cannon. Gently removing Doane's body and laying it on the ground al respectful-like, he then leapt into the firing chair and whirled it on the Alliance ship just as it finished raining down fire on the shepherd's own church and adjoining garden.

Though Bernabe didn't rightly recognize the specific class of ship, he was familiar with the design type behind it. Was fair similar to the Striker-class that the Alliance favored during the war. This was souped up beyond all good sense, but Bernabe figured the engine core was in the same place.

He knew this mainly because Book's first and only shot went right to the spot on the undercarriage where Bernabe reckoned the core was.

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This is fully confirmed by the film's screenplay

It is a world of fire.

Every building burns, some blown right apart. Bodies litter the scene, not one of them moving. There's a grounded Alliance ship not far from the cannon that shot it down.

INT. BRIDGE - CONTINUING

We see Mai's face as the sight hits him like a gut-punch. EXT. MINING CAMP - MOMENTS LATER The crew pours out even as the ramp lowers, going off in all directions, calling out to people...

Kaylee heads for the burning church.

shortly later...

MAL: Jayne, you and Wash hoist up that cannon mount. Goes right on top. Piece or two of the other ship, stick it on. Any place you can tear hull without inner breach, do that too.

(looking around) And we're gonna need paint. We're gonna need red paint.